Metal-oxide-semiconductor (MOS) devices are key components of integrated circuits. A MOS device can work in three regions, depending on gate voltage Vg and source-drain voltage Vds, linear, saturation, and sub-threshold regions. The sub-threshold region is a region where Vg is smaller than the threshold voltage Vt. A parameter known as Sub-threshold Swing (SS) represents the easiness of switching the transistor current off and thus is an important factor in determining the speed of a MOS device. The sub-threshold swing can be expressed as a function of m*kT/q, where m is a parameter related to capacitance. The sub-threshold swing of a typical MOS device has a limit of about 60 mV/decade (kT/q) at room temperature, which in turn sets a limit for further scaling of operation voltage VDD and threshold voltage Vt. This limitation is due to the diffusion transport mechanism of carriers. For this reason, existing MOS devices typically cannot switch faster than 60 mV/decade at room temperatures. The 60 mV/decade sub-threshold swing limit also applies to FinFETs or ultra thin-body MOSFETs on silicon-on-insulator (SOI) devices. However, even with better gate control over the channel, an ultra thin body MOSFET on SOI or a FinFET can only achieve close to, but not below, the limit of 60 mV/decade. With such a limit, faster switching at low operation voltages for future nanometer devices cannot be achieved.
To solve the above-discussed problem, Tunnel Field Effect Transistors (TFETs) have been explored. TFETs can improve both of these parameters by changing the carrier injection mechanism. In a MOSFET, the SS is limited by the diffusion of carriers over the source-to-channel barrier where the injection current is proportional to kT/q. Hence at room temperature, the SS is 60 mV/dec. In a TFET, injection is governed by the band-to-band tunneling from the valence band of the source to the conduction band of the channel. Accordingly, much lower sub-threshold swing can be achieved. Since the TFETs are often designed to have a p-i-n diode configuration, much lower leakage currents are achieved. Also, the TFETs are more resistant to short-channel effects commonly seen on MOSFETs.